This invention relates to a temperature compensating member having a negative coefficient of thermal expansion and an optical communication device using the same.
With the advance of the optical communication technology, a network using optical fibers has been rapidly built up. In the network, a wavelength multiplexing technique of collectively transmitting light beams having a plurality of different wavelengths has come into use, and a wavelength filter, a coupler, a waveguide, and the like have become important devices.
Some of the devices of the type described are changed in characteristics depending upon the temperature and may therefore cause troubles if used in the outdoors. This requires a technique for keeping the characteristics of these devices fixed or unchanged regardless of a temperature change, i.e., a so-called temperature compensating technique.
As a typical optical communication device which requires temperature compensation, there is a fiber Bragg grating (hereinbelow, referred to as FBG). The FBG is a device in which a portion varied in refractive index in a grid-like pattern, i.e., a so-called grating is formed within a core of an optical fiber, and has a characteristic of reflecting a light beam having a specific wavelength according to the relationship represented by the following formula (1). Therefore, the device attracts attention as an important optical device in the optical communication system using a wavelength division multiplex transmission technique in which optical signals different in wavelengths are multiplexed and transmitted through a single optical fiber.
xcex=2nxcex9xe2x80x83xe2x80x83(1)
Herein, xcex represents a reflection wavelength, n, an effective refractive index of the core, and xcex9, a grid interval of the portion varied in refractive index in the grid-like pattern.
However, the above-mentioned FBG has a problem that the reflection wavelength will be varied following the change in ambient temperature. The temperature dependency of the reflection wavelength is represented by the following formula (2) which is obtained by differentiating the formula (1) with the temperature T.                                                                                           ∂                  λ                                /                                  ∂                  T                                            =                              2                ⁢                                  {                                                                                    (                                                                              ∂                            n                                                    /                                                      ∂                            T                                                                          )                                            ⁢                      Λ                                        +                                          n                      ⁡                                              (                                                                              ∂                            Λ                                                    /                                                      ∂                            T                                                                          )                                                                              }                                                                                                        =                              2                ⁢                Λ                ⁢                                  {                                                            (                                                                        ∂                          n                                                /                                                  ∂                          T                                                                    )                                        +                                                                  n                        ⁡                                                  (                                                                                    ∂                              Λ                                                        /                                                          ∂                              T                                                                                )                                                                    /                      Λ                                                        }                                                                                        (        2        )            
The second term of the right side of the formula (2), i.e., (∂xcex9/∂T)/xcex9 corresponds to a coefficient of thermal expansion of the optical fiber and has a value approximately equal to 0.6xc3x9710xe2x88x926/xc2x0 C. On the other hand, the first term of the right side corresponds to the temperature dependency of a refractive index of the core portion of the optical fiber and has a value approximately equal to 7.5xc3x9710xe2x88x926/xc2x0 C. Thus, it will be understood that the temperature dependency of the reflection wavelength depends on both the variation in refractive index of the core portion and the change in grid interval due to thermal expansion but mostly results from the temperature-dependent variation of the refractive index.
As means for avoiding the above-mentioned variation in reflection wavelength, there is known a method in which the FBG is applied with tension depending upon the temperature change to thereby change the grid interval so that a component resulting from the variation in refractive index is cancelled.
As a specific example of the above-mentioned method, proposal is made of a method in which the FBG is fixed to a temperature compensating member which comprises a combination of a material, such as an alloy or a silica glass, having a small coefficient of thermal expansion and a metal, such as aluminum, having a large coefficient of thermal expansion. Specifically, as shown in FIG. 1, an Invar (trademark) bar 10 having a small coefficient of thermal expansion has opposite ends provided with Al brackets 11a and 11b having a relatively large coefficient of thermal expansion attached thereto, respectively. An optical fiber 13 is fixed to these brackets 11a and 11b by the use of clasps 12a and 12b so that the optical fiber is stretched under a predetermined tension. At this time, adjustment is made so that the grating portion 13a of the optical fiber 13 is located between the two clasps 12a and 12b. 
If the ambient temperature rises in the above-mentioned state, the brackets 11a and 11b are expanded to reduce the distance between the two clasps 12a and 12b so that the tension applied to the grating portion 13a of the optical fiber 13 is decreased. On the other hand, as the ambient temperature falls, the brackets 11a and 11b are contracted to increase the distance between the two clasps 12a and 12b so that the tension applied to the grating portion 13a of the optical fiber 13 is increased. Thus, by changing the tension applied to the FBG depending upon the temperature change, it is possible to adjust the grid interval of the grating portion. As a result, it is possible to cancel the temperature dependency of the reflection center wavelength.
However, the above-mentioned temperature compensating device is disadvantageous in that the structure is complicated and the handling is difficult.
As a method for solving the above-mentioned disadvantages, Japanese Unexamined Patent Publication No. 2000-503415 or Japanese Unexamined Patent Publication No. 2000-503967 discloses a method shown in FIG. 2, in which a FBG 16 is, under a tension applied by a weight 15, fixed to a glass ceramic substrate 14 having a negative coefficient of thermal expansion, by use of an adhesive 17, which substrate is obtained by heat-treating and crystallizing a raw glass material preliminarily formed into a plate shape. The tension is controlled by expansion or contraction of the glass ceramic substrate 14. In order to cancel the temperature dependency of the reflection center wavelength, it is necessary to apply a stress in a direction of contraction of the FBG when temperature rises and in a direction of expansion when temperature falls, as described above. As long as the substrate material has a negative coefficient of thermal expansion, such stress can be produced by a single component. The invention disclosed in the Japanese Unexamined Patent Publication No. 2000-503415 or the Japanese Unexamined Patent Publication No. 2000-503967 is achieved on the basis of the function and the effect mentioned above. In FIG. 2, 16a represents a grating portion.
The method disclosed in the Japanese Unexamined Patent Publication No. 2000-503415 or the Japanese Unexamined Patent Publication No. 2000-503967 is advantageous in that the structure is simple and the handling is easy because temperature compensation is achieved by a single component. However, there is a problem that the glass ceramic member used in the method is large in hysteresis of thermal expansion. The hysteresis of thermal expansion is a phenomenon in which, when a material expands or contracts following a temperature change, an expanding behavior upon temperature elevation does not coincide with that upon temperature drop.
In addition, the Japanese Unexamined Patent Publication No. 2000-503415 or the Japanese Unexamined Patent Publication No. 2000-503967 discloses a method for the purpose of diminishing the hysteresis of the glass ceramic member, in which a heat-cycle treatment is carried out at a temperature between 400 and 800xc2x0 C. to stabilize an internal structure. However, the hysteresis diminished by the method described above is unstable against a change in environment such as temperature or humidity and it is therefore difficult to maintain its initial value. Further, the above-mentioned heat treatment requires a complicated manufacturing process, resulting in a problem of a high cost.
Therefore, it is an object of the present invention to provide a temperature compensating member which is small in hysteresis of thermal expansion, high in environmental stability, and capable of being manufactured at a low cost.
It is another object of the present invention to provide an optical communication device using the above-described temperature compensating member.
In order to accomplish the above-mentioned objects, the present inventors have conducted various experiments and, as a result, found out that a temperature compensating member diminished in histeresis of thermal expansion and excellent in environmental stability is obtained by controlling the crystal structure of a polycrystalline body which forms the temperature compensating member. This leads to a proposal of the present invention.
According to one aspect of the present invention, there is provided a temperature compensating member which comprises a polycrystalline body containing, as a main crystal, one of xcex2-quartz solid solution and xcex2-eucryptite solid solution, which has a value less than 3.52 xc3x85 as an interplanar spacing of the crystal planes giving a main peak in X-ray diffraction measurement, and which has a negative coefficient of thermal expansion.
The polycrystalline body may be a sintered powder body.
The above-mentioned temperature compensating member may have a coefficient of thermal expansion of (xe2x88x9225 to xe2x88x92120)xc3x9710xe2x88x927/xc2x0 C. within a temperature range between xe2x88x9240 and 100xc2x0 C.
According to another aspect of the present invention, there is provided an optical communication device comprising the above-described temperature compensating member and an optical component having a positive coefficient of thermal expansion and fixed on one surface of the temperature compensating member.
The optical communication device may further comprise a reinforcing member adhered to the other surface of the temperature compensating member by the use of an adhesive having a low elasticity.
The reinforcing member may be a columnar member having a through-hole, and the temperature compensating member may be placed in the through-hole of the reinforcing member.
The optical component may be fixed to the temperature compensating member by the use of an adhesive which comprises an organic polymer and has a viscosity between 2500 and 100000 mPaxc2x7s at 25xc2x0 C. prior to curing and a contracting rate of 5% or less upon curing.
It is noted here that the interplanar spacing means a distance between various crystal planes in the crystals forming the polycrystalline body. The present invention is concerned with the crystal plane giving the main peak in the X-ray diffraction.